The present invention relates to a metering apparatus of an injection molding machine and, more particularly, to a metering apparatus which is mounted on an injection molding machine using a motor as a drive source for rotating a screw and supplying a back pressure by a servo motor used as a drive source for an injection mechanism in metering, thereby correctly performing metering.
In a conventional injection molding machine, a screw is rotated by a motor, or the like, during metering, and a back pressure is applied by a hydraulic piston, or the like. The resin is melted by rotation of the screw. When the molten pressure of the resin exceeds the back pressure, the screw is moved backward. When the screw reaches a metering pin, it is detected by a limit switch, or the like, and the rotation of the screw is stopped. However, even when the screw has reached the metering point and the limit switch is operated, the rotation of the screw is not stopped immediately because of the inertia of the screw rotating mechanism. As a result, the metered injection amount may be excessive. When the actual metering point is set at a position slightly before a logical metering point, considering inertia, the injection amount may be insufficient. Also, because of the preset position error of the limit switch or the operation error, high-precision metering cannot be performed. The metering error causes short shot, a sink mark, or a burr, so that a product having a good quality cannot be obtained.
In order to solve the above problem, as is disclosed in Japanese patent publication No. 52-43869, the screw rotation speed is reduced at a position slightly before the metering point in order to decrease the inertia, and the screw is stopped at the metering point. However, in this method as well, the position of the screw is detected by a limit switch. Therefore, a metering error occurs because of the preset position error and the operation error of the limit switch. Since the error amount generated by the inertia of the screw rotating mechanism differs depending on the rotation speed of the screw, high-precision metering cannot be performed unless the preset position of the limit switch is moved every time the screw rotation speed is changed.